1. Field of the Invention
This invention relates in general to reproduction technology as, for example, in printing wherein masters are commonly scanned which are unrastered or rastered with different raster width and/or screen angles.
2. Description of the Prior Art
The use of rastered offset masters for rotogravure so-called "offset rotogravure conversion" called OTC is gaining increasingly popularity and significance. The principle advantage of this method is that rastered offset positives can be more quickly and cheaply produced and corrected than the previously utilized half-tone separations for rotogravure. Proofs can be produced economically from OTC. Since the conversion conditions from offset to rotogravure are known, the printed results in rotogravure can be optimized before the expensive rotogravure form is produced by means of electronic engraving, for instance.
It is known from German Patent application P No. 32 17 752 which teaches a method for moire-free scanning wherein a diaphragm matched in shape, size and angular position to the raster of the master is employed for scanning.
This method, however, has limits when as described, for example, in "Der Polygraph", No. 18, 1968, the engraving of the rotogravure form occurs by means of the "Helio-Klischograph" manufactured by Dr. Ing. Rudolf Hell GmbH or when scanning and re-recording occurs in rotating scanners or similar machines.
With such machines, a multitude of master montages of entire magazine pages are mounted on the scanning cylinder and a plurality are covered line-wise by scanning optics device during a scanning pass. It is entirely conceivable that masters to be scanned in common or even the individual images within one page are different in terms of raster widths and/or screen angle. Unrastered page montages or individual images can also occur.
The method disclosed in German Patent Application P No. 32 17 752 cannot operate successfully because a mechanical switch-over of diaphragm shapes and diaphragm angles for matching the respective master part to be scanned is much too slow.
Conditions which are somewhat similar exist in scanners. Offset positives would also be desirable as masters in scanner technique for the above-mentioned reasons of economic feasibility. Masters having different raster width and/or screen angle can also occur during one scanning pass. For example, different screen angles definitely always exist when the four color separations of one image are to be acquired in a single scanning by the so-called multi-color method and the masters are rastered offset color separations.
It is an object of the present invention to provide a method and apparatus which allows the electron-optical scanning of unrastered or rastered masters having different raster widths, raster configurations and/or screen angles during one scanning pass such that when the rasters are added during the reproduction process a moire will not occur with one of the master rasters during recording.
Other objects, features and advantages of the invention will be readily apparent from the following description of certain preferred embodiments thereof taken in conjunction with the accompanying drawings although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure and in which: